This chapter addresses the hydrodeoxygenation of 5-hydroxymethylfurfural and levulinic acid for the production of chemicals, synthetic fuels and fuel additives. The reaction pathways for the conversion of these platform chemicals are discussed through the perspective of the effective H/C ratio concept. Even for these rather simple platform molecules, the reaction network is very complex, and apart from higher reaction rates especially the selectivity towards the desired molecules, such as 2,5-dimethylfuran, 2,5-bis(hydroxymethyl)furan, γ-valerolactone, is an important aspect. A fundamental understanding of the mechanisms accounting for the product distributions is identified as one of the future challenges. In situ spectroscopic methods, such as operando EXAFS and ATR-IR, are presented as powerful tools for the clarification of these mechanisms.